Abstract

We report measurements which give direct insight into the origins of the transparency current for λ ~5 µm In0.6Ga0.4As/In0.42Al0.58As quantum cascade lasers in the temperature range of 80-280 K. The transparency current values have been found from broadband transmission measurements through the laser waveguides under sub-threshold operating conditions. Two active region designs were compared. The active region of the first laser is based on double-LO-phonon relaxation approach, while the second device has only one lower level, without specially designed resonant LO-phonon assisted depopulation. It is shown that transparency current contributes more than 70% to the magnitude of threshold current at high temperatures for both designs.

Figures (3)

Experimental transmission spectra (taken for TM polarization and normalized for the spectra taken for TE polarization) for lasers S1 and S2 measured at 80 and 240 K for various current values below the laser thresholds. The spectra are vertically shifted for clarity where the spectra presented as the lowest were measured for the smallest current values. The absorption peaks observed at the energies higher than the laser transitions correspond to other intersubband transitions in the QCL active regions (see [5], for example).

The logarithm (ln(ITM/TE)/L) of the intensities of the amplification peaks (upper curves, ▲) and the absorption dips (lower curves, ●), extracted from the TM/TE transmission spectra and normalized by the cavity length L, versus the current density for QCLs S1 (a) and S2 (b). The horizontal and vertical thick black lines show the example on how the transparency current for laser S2 was estimated from the data at 240 K.